1. Field of the Invention
The invention concerns a spectral analysis unit with a diffraction grating in which a parallel light bundle which has a wavelength range falls on a diffraction grating which splits the different wavelengths through diffraction in first spectral directions, wherein this light bundle is designated as an uncycled light bundle of a first order of diffraction (“uncycled first order light bundle”), and the diffraction grating bends the light bundle in a second direction, wherein this light bundle is designated as an uncycled light bundle of zero order of diffraction (“uncycled zero order light bundle”), furthermore parts of the wavelength range of the spectrally split uncycled first order light bundle can be focused on a detector line (3) through optics and evaluation electronics is connected to the detector line which receives and displays the created spectrum as data. The spectral unit finds application in all spectrometers. In particular the unit comes in use in a confocal Laser Scanning Microscope (LSM), such as the one described in DE 197 02 753 A1 or U.S. Pat. No. 7,009,699 B2, as measuring equipment for the spectrally split detection of fluorescence.
2. Related Art
The spectral unit with a diffraction grating is built in principle as a Spectrograph. A broad range light radiated from a sample is broken down spectrally through a dispersive element and then is measured by means of a detection unit and evaluated. In this way, a diffraction grating is set up as a dispersive element. Principally diffraction gratings are differentiated according to levels of flat grating with equidistant lines and a picturing grating which is preferably created holographically.
With the levels flat grating, collimation optics between the grating and the light source and a focusing optics between the grating and spectrum are necessary. These optics can be lenses or mirrors. Normally the spectrum of the first order of diffraction of the diffraction grating is created using the focusing optics on the receiver of the detection unit. The problem is that the diffraction efficiency of the diffraction grating changes very markedly depending on the wavelength, the grating constants, the grating material and the profile form i.e., the complete transmission has strong limitations because of physical conditions.
In particular with small grating constants (g) further polarization effects appear if g is of the order of magnitude of the wave length or is smaller. The polarization effects show themselves in a strong split according to the intensity of the TE and TM polarization, by which the diffraction intensity is strongly reduced at least in the border ranges of the spectrum.
With a mechanically created blazed grating it is common to determine the blaze angle of grating grooves such that for a certain desired wavelength, highest possible diffraction efficiency is reached or a compromise for the fall in diffraction efficiency is created. The bigger the spectral range, the bigger the fall.
A known method for getting around this problem is shown by the use of Echelle systems such as is described in U.S. Pat. No. 5,189,486. Here a flat grating is used in very high diffraction orders whereby at first short overlapping spectral areas come up all of which work in the neighborhood of the blaze angle. For lateral separation of the spectrum a prism is added in the system which works perpendicularly to the dispersion direction of the grating. Through this one gets many lateral overlapping order lines. However the condition for the use of this process is the use of a surface receiver.
The reason for the spectral variation of the efficiency of diffraction lies in that the electromagnetic behavior of the grating diffracts one more or less big part of the light in other than the desired first order of diffraction, through which it is lost and even further creates scattered light problems. In particular the biggest part of the energy in the zero order of diffraction gets lost, particularly then when no further bigger first orders of diffraction or no smaller zero order of diffraction can appear physically.
The invention is intended to solve the task to increase significantly the diffraction efficiency of a spectral analysis unit with a grating with small expense.